In a mounting substrate used in modern computers mounting a number of LSIs very densely, the wiring patterns are so fine that defects such as short-circuits among the wirings or breaking of the wires are likely to easily develop.
The wiring test for such mounting substrates has, so far, simply consisted of checking the wire connection between points such as between output pins of the individual LSIs and input pins of other LSIs connected thereto, and checking the signal levels, but did not include checking for short-circuits and defects among the wirings. This is because the signal levels are quite different depending upon the output pins of the LSIs and it is difficult to determine abnormal conditions by specifying the levels produced by the wiring short-circuits. This will be described below with reference to a concrete example shown in FIG. 13, wherein reference numeral 1 denotes circuit-mounting substrates or LSI logic circuits mounting LSIA to LSIE, and reference numerals 1.sub.1 to 1.sub.6 denote wirings.
Here, for instance, a break in the wiring 1.sub.2 can be detected based on the fact that the output level of the LSIB to 1.sub.2 is not in agreement with the input level to the LSIE or that the input level of the LSIE fails to follow a change in the output level of the LSIB. Moreover, the short-circuit between 1.sub.4 and V.sub.EE can be detected based on the fact that the level of V.sub.EE becomes abnormal. However, the short-circuit or the defect between 1.sub.5 and 1.sub.6 cannot be detected when the 1.sub.5 output level of the LSIC and the 1.sub.6 output level of the LSID are both "1" or "0". Therefore, the logic outputs of the LSIs for 1.sub.5 and 1.sub.6 must be made different. That is, the short circuited condition is discriminated based on an intermediate level which is created by the combination of different levels of the two wirings. When there are a large number of wirings, therefore, there exist a tremendous number of combinations of the wirings that need checking for short-circuits, and it becomes very difficult to control the logic outputs.
The object of the present invention is to provide means which facilitates the detection of short-circuits among the wirings in testing the wirings of a substrate on which are mounted LSI logic circuits.